PPC management agencies offer many different pricing models. Most agencies charge between 10-25% of your ad spend, and the industry standard rate is 15% of ad spend. Some offer fixed price packages, those package prices are usually based on an ad spend limit that works out to be about the same as industry rates. It's generally a good idea to go with an agency that charges close to industry standard rates, if they are lower they will need to cut corners to stay within the fee budget which tends to affecting the quality of your service, and if they charge too high it eats the majority of your profits. In my opinion, it is also better to go with an agency that offers variable rates rather than fixed, else they have a built-in incentive to limit the growth of your business to keep their own expenses within the fixed fee. In the interest of full disclosure, I operate an advertising agency so I will recuse myself from recommending a specific agency. Advertising your business is essential and is not something to take lightly. Not all people or even agencies have the same knowledge levels. It's all about results. It may be worth to spend 20% instead of 10% if the more expensive one produces much better results.